Particulates are tiny clumps of soot, dirt, and various chemicals that have been linked to a wide variety of health problemsxe2x80x94asthma, and higher rates of disease affecting the cardiovascular system or lungs. Since 1987, EPA standards have governed all particulates under 10 micrometers in diameter. This category of particulate matter is called PM10. Recently, however, studies have suggested that the most dangerous particles are actually the smaller ones, which penetrate deeper in the lungs"" aereoles. Thus, new regulations will build in a separate standard for particles less than 2.5 micrometers in diameterxe2x80x94PM2.5.
While PM10 contains a lot of wind-blown soil, PM2.5 is derived mainly from burning fossil fuels. PM2.5 typically contains a mixture of elemental carbon, organic carbon, sulfate and nitrate particles, and acid droplets. It is unlikely that all components of PM2.5 contribute equally to the observed health effects, yet the present lack of sufficient data quantifying the individual components prevents the EPA from separately regulating these components. Because regulating PM2.5 collectively is not a cost-effective solution, the agency is under great scientific, industrial, and political pressure to specifically identify sources of the observed particle health-effects. Thus, interest in measuring the individual components of PM2.5 has increased dramatically over the last few years.
A number of methods are known for measuring atmospheric nitrate levels. Koutrakis et al., Environ. Sci. Technol. 22:1463, 1988 disclose an integrated sampling method (Harvard/EPA Annular Denuder System (HEADS)) which is designed to measure various atmospheric components including particulate nitrate. The method provides a non-quantitative conversion of particulate nitrate to nitric acid vapor by collection of atmospheric fine particles on a Teflon filter, with a sodium carbonate-coated filter downstream to collect nitric acid vapor produced by volatization of ammonium nitrate and by the reaction of ammonium nitrate with acidic sulfate particles.
Wendt et al., xe2x80x9cContinuous monitoring of atmospheric nitric oxide and nitrogen dioxide by chemiluminescencexe2x80x9d in Methods of Air Sampling and Analysis, editor, J. P. Lodge Jr., Lewis Publishers, Chelsea, Mich., pp 415-421 (1989), disclose a continuous chemiluminescent NOx detection method. Yamamoto et al., Anal. Chem., 1994, 66, 362-367, describe a nitrate analysis method relying on chemiluminescent NOx detection. NOx generally refers to NO2 and NO taken together.
Brauer et al., Environ. Sci. Technol. 24:1521, 1990 disclose a method for the continuous measurement of nitrous acid and nitric acid vapors which does not distinguish between the two species. Klockow et al., Atmospheric Environment, 1989, 23, 1131-1138, disclose thermodenuder systems for the discontinuous measurement of nitric acid vapor and ammonium nitrate. Buhr et al., Atmospheric Environment, 1995, 29, 2609-2624, teach a denuder for sampling nitric acid, nitrate, and sulfate. Wolfson et al., U.S. Pat. No. 5,854,077, present a continuous differential nitrate measurement method.
Many of these and other existing methods for nitrate measurement require labor-intensive, manual collection of 24-hour integrated samples and laboratory analysis of the collected components. Not only are such samples expensive to collect, but the lengthy collection period prevents the detection of cycles and patterns which occur over the course of a day. Convenient techniques which offer improved temporal resolution and are capable of unifying the collection and analysis processes are badly needed now to reveal these daily patterns, both for epidemiological research and for regulatory monitoring.
The systems and methods described herein relate to the measurement of nitrate in gas samples by collection and analyzing samples by a technique which permits a short cycling time. Thus, in one aspect, the invention provides a system for measuring nitrate levels having a sample inlet for receiving a sample of gas, a collection body coupled to said sample inlet, a filter mounted within said body to collect particles from said sample of gas, a heater coupled to the body to heat the body, a gas inlet coupled to said body to provide a flow of gas through said body, and a detector coupled to said body to measure an NOx concentration. In a certain embodiment, the system further comprises a source of gas coupled to said gas inlet. The gas may be nitrogen or another gas which is substantially free of oxygen.
In another embodiment, the system also includes a catalyst, coupled to said body and to said detector, capable of reducing NO2 to NO. The catalyst may comprise molybdenum, carbon, or ferrous sulfate.
In certain embodiments, the detector included in the system has a light sensor, and may further include an ozone generator, for example, for the detection of the chemiluminescent oxidation of NO. In another embodiment, the detector includes an infrared sensor. In yet another embodiment, the detector includes a material which reversibly binds NO.
In one embodiment, the filter comprises quartz fibers.
In yet another embodiment, the system includes an extractor coupled to the sample inlet and to the collection body to substantially remove NO2 from the gas sample. The extractor may comprise a hydroxyl-bearing solvent and a base, e.g., glycerol and an organic base, e.g., an amine, such as triethanolamine.
In yet another embodiment, the system also includes a selection platform, situated between the sample inlet and the extractor, to substantially remove particles larger than about 2.5 microns. The selection platform may be a filter, an inertial impactor, or any other suitable device.
In one embodiment, the system further includes a cooling system to cool the collection body.
In yet another aspect, the invention relates to a method for measuring a level of nitrate by receiving a gas sample, collecting nitrate particles from the gas sample on a filter, passing a stream of gas substantially free of oxygen over the collected particles, volatilizing the collected particles by heating to generate NOx, and measuring a level of NOx.
In one embodiment, the method further includes substantially removing NO2 prior to collecting nitrate particles, e.g., by passing the received sample over a hydroxyl-bearing solvent and a base, e.g., an organic base such as triethanolamine.
In another embodiment, the method further includes removing particles larger than about 2.5 microns from the received gas sample, e.g., by passing the received sample through an inertial impactor or by passing the received sample through a filter.
In one embodiment of the method, passing a stream of gas includes passing a stream of nitrogen over the collected particles.
In yet another embodiment, the method further comprises reducing generated NO2 to NO using a metal catalyst, e.g., by contacting the NO2 with a molybdenum catalyst.
In certain embodiments, measuring a level of NOx includes reacting NO with ozone. In yet another embodiment, measuring a level of NOx includes detecting infrared absorption. In certain other embodiments, measuring a level of NOx includes adsorbing NOx on a conductive material.
In one embodiment, collecting nitrate particles comprises collecting nitrate particles on a filter comprising quartz fibers.
In another embodiment, volatilizing the collected particles includes rapidly heating the collected particles to at least 300xc2x0 C.
In yet another aspect, the invention provides a system for measuring nitrate levels, including a sample inlet to receive a sample of gas, an extractor coupled to said sample inlet to substantially remove NO2 from the gas sample, a collection body coupled to said sample inlet, an inertial impactor mounted within said body to collect particles from the gas sample, a current source coupled to the inertial impactor to heat the inertial impactor and generate NOx and a detector coupled to said catalyst to measure an NOx concentration.
In yet another aspect, the invention relates to a method for measuring a level of nitrate by receiving a gas sample, substantially removing NO2 from the gas sample, collecting nitrate particles from the gas sample with an inertial impactor, passing a stream of gas substantially free of oxygen over the collected particles, volatilizing the collected particles by heating to generate NOx and measuring a level of NOx generated by the heated particles.
In yet another aspect, the invention provides a system for measuring nitrate levels having means for receiving a sample of gas, support means coupled to the means for receiving, means for collecting particles coupled to the support means, means coupled to the support means, for heating the support means to generate NOx, means, coupled to the support means, for flowing a stream of gas through the support means, and means for measuring an NOx concentration coupled to the support means.
In one embodiment, such a system also includes means for substantially removing NO2 from the sample of gas, coupled to said means for receiving and said support means.
In another embodiment, such a system further includes means for reducing NO2 to NO, coupled to the support means and to the means for measuring.
In yet another aspect, the invention relates to a method of manufacturing a nitrate measurement apparatus by providing a sample inlet for receiving a sample of gas, coupling a collection body to the sample inlet, disposing a filter within the body, coupling a heater to the body, coupling a gas inlet to the body, and coupling an NOx detector to the body.
In one embodiment, the method further comprises disposing an NO2 extractor between said sample inlet and said collection body.
In another embodiment, the invention further comprises disposing a catalyst capable of reducing NO2 to NO between said collection body and said NOx detector.